Conventionally, a solder paste has mainly been used when soldering an electronic component such as a chip component on a printed board in manufacturing an electronic circuit board, and a solder paste printing apparatus is used for printing this solder paste in the desired pattern. As one example of the squeegee head mounted on a conventional solder paste printing apparatus, a construction as shown in FIG. 13 can be enumerated.
Normally, during a printing operation, a squeegee head 102 reciprocates from the left to the right and from the right to the left in FIG. 13 every printed board 5. In this case, a rightward direction printing squeegee 101a is used for the rightward direction printing from the left to the right, and a leftward direction printing squeegee 101b is used for the opposite leftward direction printing.
The solder paste printing operation on the printed board 5 by the conventional solder paste printing apparatus 100 will be described with reference to FIG. 13 through FIG. 15.
In FIG. 13 through FIG. 15, reference numeral 3 denotes a printing mask through which openings 4 of the desired pattern are formed, 5 a printed board, 6 a land on which a solder paste 7 is to be printed, and 8 a solder resist. The desired pattern of the printing mask 3 means a pattern on which the openings 4 are formed in correspondence with the lands 6 on the printed board 5.
First of all, when performing the rightward direction printing, the printed board 5 is positioned when the printing mask 3 in layers so that the openings 4 and the lands 6 are aligned in position with each other, and thereafter, the rightward direction printing squeegee 101a is moved down in a state in which the leftward direction printing squeegee 101b is moved up, then bringing a squeegee edge portion 103 in contact with a surface 3a of the printing mask 3 with an appropriate pressure applied.
In this state, by linearly moving the rightward direction printing squeegee 101a in the rightward direction, the solder paste 7 preparatorily provided on the surface 3a of the printing mask 3 is gradually filled into the openings 4 of the printing mask 3. The printing operation is ended by moving the rightward direction printing squeegee 101a to the right end of the printing mask 3 and thereafter separating the printed board 5 from the printing mask 3.
When performing the leftward direction printing, similarly to the rightward direction printing, the printed board 5 is positioned under the printing mask 3 in layers, and thereafter, the leftward direction printing squeegee 101b is moved down with the rightward direction printing squeegee 101a oppositely moved up this time, then bringing the squeegee edge portion 103 in contact. The subsequent operation is similar to that of the rightward direction printing described hereinabove.
Thus, by alternately repeating these operations every printed board 5, the solder paste 7 is continuously printed on the lands 6 of each printed board 5 via the printing mask 3 as shown in FIG. 16.
According to the printing using the conventional solder paste printing apparatus 100, the printing squeegee 101a or the printing squeegee 101b is moved in the state in which the squeegee edge portion 103 of the printing squeegee 101a or the printing squeegee 101b is brought in contact with the surface 3a of the printing mask 3 with the appropriate pressure applied. As is apparent from the above, the conventional printing squeegees 101a and 101b perform the two operations of a scraping operation for scraping the solder paste 7 off the surface 3a of the printing mask 3 and a filling operation for filling the solder paste 7 into the openings 4 of the printing mask 3 by the squeegees of one type. This will be described with reference to FIG. 14 and FIG. 15.
FIG. 14 and FIG. 15 are enlarged views of a printing squeegee 101a and others for the rightward direction printing. First, as shown in FIG. 14, if the rightward direction printing squeegee 101a is moved down and linearly moved in the rightward direction so that the edge portion 103 comes in contact with the surface 3a of the printing mask 3, then the rightward direction printing squeegee 101a reaches the solder paste 7 supplied to the surface 3a of the printing mask 3 and moves while scraping off the paste.
By this scraping operation, the solder paste 7 flows with a rotational motion called the rolling as indicated by arrow I in FIG. 15. At this time, a fluid pressure is generated inside the solder paste 7.
When the rightward direction printing squeegee 101a moves further to the right in the above-mentioned state and reaches the openings 4 of the printing mask 3, the solder paste 7 is pushed into the openings 4 by the above-mentioned fluid pressure, and the so-called filling of the solder paste 7 is achieved. Hereinafter, the pressure by which the solder paste 7 is pushed into the openings 4 will be referred to as a filling pressure.
On the other hand, from the point of view of an improvement in productivity, it is desired to reduce the printing time even in the solder paste printing process. However, if the travel speed (squeegee speed) of the squeegee 101a is increased in the conventional solder paste printing apparatus 100 for the reduction of the above-mentioned time, the so-called unfilled portion 9, where the amount of the solder paste 7 filled into the openings 4 of the printing mask 3 becomes insufficient, is generated as shown in FIG. 17, causing defective printing. As shown in FIG. 18, there occurs lacks of the solder paste 7 printed on the lands 6 of the printed board 5, and stable printing cannot be performed. The reasons are as follows.
If the squeegee speed is increased, then the time during which the edge portion 103 of the printing squeegee 101a passes over the openings 4 becomes reduced. Therefore, the time during which the solder paste 7 is filled into the openings 4 (hereinafter, referred to as a filling time) is, of course, reduced.
In order to examine this phenomenon, as shown in FIG. 17, there was measured the filling pressure when a pressure sensor 51 was arranged on the back surface of the printing mask 3 provided with an opening 4a located in the corresponding portion of the mask and the squeegee 101a was moved at a travel speed of 40 mm/sec and 200 mm/sec. FIG. 19 is a graph showing the measurement results.
The time t along the horizontal axis is the time during which the squeegee 101a passes over the pressure sensor 51, and the filling pressure P is the pressure that the pressure sensor 51 has detected via the solder paste 7 when the squeegee 101a passes over the pressure sensor 51.
By this graph, it is clearly understood that the filling time is reduced when the travel speed of the squeegee 101a is increased.
Moreover, although the filling pressure increases itself when the squeegee speed is increased, the time during which the edge portion 103 of the squeegee passes over the openings 4 is reduced, and further the duration of the high pressure is short. Therefore, a sufficient filling time cannot be obtained. As a result, the unfilled portion 9 is disadvantageously generated.
As described above, the conventional solder paste printing apparatus has not been able to achieve both of an increase in the printing speed and stable printing.
The object of the present invention is to solve the aforementioned conventional issues and provide a solder paste printing apparatus and printing method capable of performing stable printing even when the printing time is reduced for an increase in speed and consequently achieving high-quality solder paste printing with high productivity.